The present disclosure relates generally to avalanche photo detector devices, and more specifically to the structure and method of making a radiation and temperature hard avalanche photodiode device.
Among the available compound semiconductor materials, III-Nitride materials, AlGaN, GaN, and InGaN are chosen as the best candidates for the development of radiation hard Avalanche Photodiodes (APDs). Recent advances in GaN power electronics and high electron mobility transistor technologies allow for future fabrication of transistor gates directly on high quality GaN wafers. This would provide for a much more robust radiation hard technology with high data capture speeds.
Micro channel plates (MCPs), micro sphere plates (MSPs), and multi-dynodes are representatives of the most established detector technology class. In particular, MCPs are widely utilized to detect photons, electrons, or ions in numerous application areas. Photon detection is necessary for astronomical observatories, fluorescent microscopy, night vision, security monitoring, and nuclear scintillation detection. Time of flight (TOF) mass spectrometry is a major tool in charged particle detection and is currently used in many applications including residual gas monitoring, bio-mass spectrometry, aerosol monitoring, and plasma sampling.
Photomultiplier tubes (PMT) have also served as excellent photon counting devices with sensitivity ranging from UV to IR. The efficiency of PMTs is typically in the range of 10% to 40% and is limited by the efficiency of the photoelectron emission from the photocathode. The dark-count events can be as low as few events per second, particularly if the PMT is cooled. The challenges of working with PMTs are that they require vacuum in order to operate and even low resolution imaging arrays are bulky, mechanically and thermally unstable, and expensive. The work principles of a single photon avalanche photodiode (SPAD) is very similar to the PMT: the incident photon generates electron-hole pairs, which are then accelerated under a high electric field in the avalanche region, where they undergo impact ionization and multiply. Thus high ionization rates lead to higher probability of avalanche which in turn improves the photon detection efficiency (PDE).
The APD structure could be grown on silicon wafers to take advantage of direct integration with established silicon technology. However, silicon is prone to false events under high radiation environments. In addition, fabricating transistor gates prior to growing the APD structures would not be possible, due to the high growth temperatures of III-Nitrides.
The benefits offered by the employment of AlGaN material for avalanche ionization based devices have been recently explored. The calculated electron and hole ionization coefficients for AlxGa1-xN materials are shown in FIG. 1. While the electron and hole ionization coefficients are very close for GaN, increasing the Al composition in AlxGa1-xN shows drastic increase of the electron ionization coefficients over the hole ionization coefficients. While electron and hole ionization are equally important for fast response of avalanche photodiodes, slight differences for improvement of the reset time in Geiger-mode avalanche photodiodes are expected at higher electron ionization coefficients. In addition, since AlGaN bandgap is higher than that of GaN and is increasing with the increase of the Al content, employment of AlGaN is beneficial as band-to-band tunneling currents are expected to reduce with increasing bandgap.
Several groups have fabricated GaN and AlN avalanche photodiodes with dark currents below 10−11 A at −150 V for AlN APD and below 10−7 A at −60 V for GaN APD. The advantages of AlxGa1-xN materials have already been demonstrated in solar-blind AlGaN p-i-n photodiodes with dark current densities as low as 3×10−11 A/cm2 with reverse breakdown voltages exceeding 40 V. The low dark currents are attributed to lower defect density and the wide bandgap of Al0.45Ga0.55N absorber layer.
Nevertheless, obtaining high quality III-Nitride layers with low defect density is still a challenge due to the lack of low cost lattice matched substrates. Several groups have demonstrated high quality GaN films grown on sapphire substrates using techniques, such as epitaxial layer overgrowth (ELO) and selective area growth (SAG), with the lowest defect densities ranging between 109 to 106 cm−2. Many devices including light emitting diodes (LEDs) have benefited significantly from such techniques. Recent developments in nanotechnology, specifically nanowires or nanocolumns, have shown the possibility to reduce the defect densities further.
It is expected that the III-Nitride nanocolumns could be defect free due to the strain management in the one-dimensional structure, allowing for the possibility of fabricating devices that approach their theoretical efficiency.
In the case of avalanche photodiodes, the zero defect densities will result in lower dark current from reduced tunneling and thermal currents. Typically 2D films of AlxGa1-xN grown on silicon have an inherent defect density exceeding 106 cm−2 Recently, several groups have shown well aligned stress free single crystalline III-Nitride nanowires grown spontaneously on silicon. The nanowires are generated due to lattice mismatch strain energy between the III-Nitride nanowires and the silicon substrates, and the high surface energy of the nitrogen stabilized InN and GaN surfaces. Defect free AlxGa1-xN nanowires have been demonstrated by Allah et al., and room temperature LED devices emitting at 1.46 μm were demonstrated on GaN nanocolumns capped with InGaN p-n junction LEDs. TEM cross-sectional images do not show any visual defects and photoluminescence studies indicate possible defect free films for these nanocolumns.
The generally accepted growth mechanism of the III-Nitride nanowires is by side wall diffusion of the adatoms during the growth process. There are two stages to the growth of the nano columns, first the nucleation stage of GaN islands followed by the growth of the GaN nanowire. The nucleation stage determines the nanocolumn size and density, which is in turn controlled by the adatom flux arriving at the substrate and the substrate temperature. Substrate temperature controls the adatom diffusion length both on the substrate and along the nanowire sidewalls. Thus it has been shown that the nanowire separation is about the twice the diffusion length of the adatom on the substrate. The second stage is the vertical growth of the III-Nitride nanowire, and again is dependent on the adatom diffusion. For instance in the case of nanowires that started off as GaN and later switched to AlGaN, it was noted that the diameter of the nanowires increased. Thus while not trivial, it is possible to control the diameter of the GaN or AlGaN nanowires by either varying the adatom flux density or varying the temperature of the substrate. The control over the nanocolumn diameter during the growth makes it possible to realize conditions when nanocolumns coalesce, forming a gap free mushroom like surface.